This invention relates to a surface reforming method for reforming the surface of an aluminum alloy member to improve its fatigue strength without causing damage to surface roughness.
In recent years, there is a demand for lightweight automobiles, which requires many aluminum alloy members to be used as the reinforcement member in a lightweight vehicle. Various attempts to improve the fatigue strength (durability) of aluminum alloy members have been made, accordingly. For example, this problem is approached by the improvements of alloy ingredients or heat treatments of the aluminum alloy member.
There is meanwhile a technique known as plastic working wherein compression residual stress is generated on the surface of an aluminum alloy member to improve its fatigue strength. Among a variety of plastic working, shot peening treatment is regarded as one of the effective methods for generating compression residual stress available for the improvements of fatigue strength.
The conventional shot peening treatments of generating compression residual stress on the surface of an aluminum alloy member have been carried out at room temperature. To improve the fatigue strength of the aluminum alloy member by compression residual stress, various studies have been made not only on the quality and particle size of shot members, but also on the conditions of the shot peening treatment such as shot peening pressure.
A hard shot peening treatment may not be applicable to members such as aluminum alloy members having a low strength along with a high brittleness because of the occurrence of rough surface and surface peeling. Due to these disadvantageous effects, it has been difficult to generate a high compression residual stress on the surface of the aluminum alloy member.
An aluminum alloy member, namely JIS-AC4C (Al-Si-Mg) is treated through a shot peening treatment at room temperature in the arc height range of 0.05 mm to 0.4 mm. FIG. 6 shows the relationship between the compression residual stress caused by the shot peening treatment and the resulting surface roughness of the aluminum alloy member (i.e., JIS-AC4C (Al-Si-Mg)).
As shown in FIG. 6, for the shot peening treatment carried out at room temperature, the surface of the aluminum alloy member tends to become rougher with the increase of the compression residual stress, or the arc height. In other words, the generation of high compression residual stresses to obtain a superior fatigue strength inevitably leads to the occurrence of a severe rough surface.
Japanese published Patent Application 1-208415 discloses a technique relating to plastic working such as shot peening treatment. In accordance with this prior technique, the surface of a casting is rapidly molten and then is rapidly resolidified, thereafter the resolidified surface being treated by a shot peening treatment. This prior art, however, has a drawback in the application to aluminum alloy members. Since aluminum alloy members have a good thermal conductivity, it is difficult to equalize the thickness of resolidified layers which are formed in the processes of rapid melting and re-solidification. Additionally, the occurrence of rough surface described above cannot be avoided because the shot peeing treatment of the resolidified layer is carried out after a cooling process.